The present invention relates to a magnetic refrigerating apparatus improved in refrigerating efficiency.
Magnetic refrigerating apparatuses which utilize magnetocaloric effect are conventionally known. In these magnetic refrigerating apparatuses, an object to be cooled is deprived of heat by means of a magnetic substance which is cooled by heat insulation and demagnetization. These apparatuses have an advantage over ordinary gas refrigerating apparatuses in being higher in refrigerating capability per unit volume.
The magnetic refrigerating apparatuses require two heat exchange processes to be performed alternately. One of these processes is a heat radiation process in which a magnetic substance or working substance represented by gadolinium-gallium-garnet is rapidly introduced into a magnetic field to be magnetized and heat produced in the working substance is discharged to the outside, while the other is an endothermic process in which the working substance located inside the magnetic field is quickly removed from the magnetic field to be demagnetized so that the object to be cooled is cooled by endothermic reaction of the working substance. Namely, the working substance must be located alternately inside and outside the magnetic field.
To attain this, in the prior art apparatuses of this type, a magnetic field generating device formed of a superconductive coil is provided around the working substance which is rigidly held in position. In adiabatic magnetization, the magnetic field generating device is energized, and a heat radiation system is actuated. In adiabatic demagnetization, on the other hand, the magnetic field generating device is deenergized, and the operation of the heat radiation system is stopped. These processes are repeated alternately.
This arrangement provides advantages such as the apparatus can be miniaturized because a refrigerating cycle can be executed by electrical control only, and the heat radiation process can be improved in reliability since the working substance is stationary.
However, since the magnetic field generating device or superconductive coil is energized pulsatively, energy loss is great, that is, refrigerating efficiency is low.
In order to eliminate such a drawback, a method is proposed in which the magnetic field generating device is energized at all times, and the working substance is mechanically moved so as to be located alternately inside and outside a magnetic field generated by the magnetic field generating device. In this case, it is not advisable to provide a long traveling stroke for the working substance, since a longer stroke will lead to an increase in the overall size of the apparatus. Thereupon, the traveling stroke of the working substance may be shortened by eliminating the so-called low-intensity magnetic field region at the peripheral edge portion of the magnetic field and by reducing the distance between a high-intensity magnetic field region and a region without magnetism. Such an arrangement would, however, result in the following problem. Even though a portion of the working substance is located inside the nonmagnetic region, the remaining portion will always be located inside the high-intensity magnetic field region as the working substance moves, since the high-intensity magnetic field region exists in close vicinity to the nonmagnetic region. If located in the region without magnetism, the working substance will be quickly lowered in temperature. In the aforesaid state, however, part of the working substance is located inside the high-intensity magnetic field region to generate heat therein. The heat from the heat generating portion moves to the low-temperature portion, so that most of the energy is consumed in the heat transfer inside the working substance. Thus, the refrigerating efficiency cannot satisfactorily be improved.